This application claims priority under 35 USC xc2xa7119 to Japanese Patent Application No. 11-341252, filed on Nov. 30, 1999, the entire contents of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to an optical writing apparatus employed in an image forming apparatus, such as laser printer, digital copying machine, facsimile device, etc., and in particular to an optical writing apparatus in which an optical writing operation is performed by use of a semiconductor laser (e.g., laser diode) array.
2. Discussion of the Background
Recently, in an image forming apparatus, such as the digital copying machine, etc., an optical writing apparatus is provided. Such writing apparatus typically includes a single laser chip constructed with a light-emitting element (e.g., semiconductor laser), a light-receiving element (e.g., photodiode) and a semiconductor laser drive controlling circuit (e.g., a semiconductor laser drive controlling device). The semiconductor laser drive controlling circuit is used for driving the light-emitting element in the laser chip and causing the light-emitting element to perform the writing operation on a line-by-line basis.
In such an optical writing apparatus, the laser beam emitted from the light-emitting element in the single laser chip is periodically deflected by use of a polygon mirror rotated by a polygon motor. The surface of a photosensitive body rotated in a sub-scanning direction is scanned in a main scanning direction and image data is written on the surface thereof.
In the semiconductor laser drive controlling circuit (LD drive controlling circuit), a negative feedback loop is connected to the laser chip. Namely, a light-receiving signal output from the light-receiving element corresponding to the light-emitting level of the light-receiving element. An APC operation is performed such that the light-emitting amount of the light-emitting element is controlled to a constant value in accordance with the light-receiving signal output from the light-receiving element in the laser chip corresponding to the light-emitting level of the light-emitting element. In other words, the normal-direction current flowing through the light-emitting element is controlled so as to equalize the light-receiving signal output from the light-receiving element corresponding to the light-emitting level of the light-emitting element.
In the recent years, increasing of print speed (i.e., image forming speed), image quality, etc., has been desired. Accordingly, high-speed rotation of the polygon motor and high frequency image formation has been attempted. However, there are respective limitations in the speeding-up of such factors. In addition, it is difficult to technically achieve such results.
In order to address such the problems, the speeding-up of such factors has been implemented by utilizing a multiple beam laser. As one such example, an optical writing apparatus is provided with plural laser chips and plural LD drive controlling circuit for simultaneously driving the light-emitting elements in the respective laser chips and causing the light-emitting elements to perform the optical writing operation for plural lines. In such a structure, the images of the plural lines are simultaneously written on the surface of the photosensitive body.
In the above apparatus, the laser beams emitted from the light-emitting elements in the respective laser chips are respectively deflected periodically by use of a polygon mirror rotated by a polygon motor. In such a method, a positional difference is caused in the sub-scanning direction on the surface of the photosensitive body rotated in the sub-scanning direction, and the surface thereof is scanned in the main scanning direction. At this time, the image is written on the surface thereof by use of the multiple beams. According to the method of writing the image by use of such the multiple beams, the amount of the image, which can be written at the same time is increased, and thereby the number of revolutions of the polygon motor and the image frequency can be reduced. Consequently, the image can be processed with high speed and in a stable manner.
However, according to the method of writing the image by use of the aforementioned multiple beams, although the print speed is further increased and the image quality is further improved, the number of the laser chips has to be increased. In addition, the structural parts may become further complicated in order to adjust the pitch in the sub-scanning direction of the light-emitting point of the light-emitting elements in the respective laser chips. Furthermore, with such an apparatus, it is necessary to further raise up the speed of the synchronization detecting plate (e.g., synchronization detecting sensor) generating and outputting the synchronization detecting signal for prescribing the writing starting position (e.g., image starting position) in the main scanning direction by the image signal of each channel (CH) for modulating and driving (e.g., turning on/off) the respective light-emitting elements.
In order to realize further speed-up of printing and further improvement of the image quality, there has been proposed an LD array (e.g., a semiconductor laser array), which is a laser chip constructed with plural light-emitting elements and a single light-receiving element. In such an LD array, plural light-emitting elements are arranged in the main scanning direction (e.g., at almost a same position in the main scanning direction) with a predetermined pitch (e.g., at equal intervals). Since the adjustment between the pitches of the light-emitting points of the light-emitting elements is determined by the property of the lens in the optical system, it is possible to omit the complicated adjustment mechanism for adjusting the rotation, etc.
In addition, since the plural light-emitting elements are arranged in a row in the main scanning direction, a synchronization detection signal of either one of channels (CH) output from the synchronization plate can be commonly used. Therefore, a phase between the respective channels (CH) does not have to be controlled. Consequently, the problem to be solved of the synchronization detecting plate, that is, the problem of the response speed thereof can be eliminated.
The background art optical writing devices have been described above. However, according to such background art devices, there exists no advantageous functional effect for improving the above-mentioned optical writing apparatus. Accordingly, the present invention has been made in view of the above-discussed and other problems and solves the above-mentioned defects and problems of the background art devices.
Specifically, an object of the present invention is to provide an optical scanning apparatus that corrects the defects and problems of the background art devices. In this respect, since the LD array is of a peculiar structure composed of plural light-emitting elements and a single light-receiving element, there does not exist an LD drive controlling circuit exclusively used for such an LD array. Accordingly, when the LD array is employed, it is necessary to construct the control circuit in a discrete state.
In addition, at the present time, a cathode-common type LD array is typically used. Furthermore, as the LD drive controlling circuit for a cathode-common type LD array of one CH (e.g., an LD driver for use in one CH), many ICs including an APC function block (e.g., a circuit for performing an APC operation) have been proposed. The present invention has been made in consideration of the above-noted matters. In the optical writing apparatus, the LD array can be driven by use of a commonly-used existing LD drive controlling circuit, such as the LD driver for use in one CH (e.g., including the APC function block).
It is another object of the present invention to simplify the entire control circuit, stabilize the LD driving by use of the LD driving IC (e.g., an exclusively-used IC), and reduce the cost thereof.
In order to attain the above and other objects of the present invention, a first aspect of the present invention provides a novel optical writing apparatus, including a semiconductor laser array constructed with plural light-emitting elements and a single light-receiving element; and plural semiconductor laser drive controlling devices respectively and simultaneously driving the plural light-emitting elements in the semiconductor laser array and thereby causing the light-emitting elements to perform the optical writing operation at the same time per each of the plural lines. The plural semiconductor laser drive controlling devices respectively include a current controlling device controlling the normal-direction currents flowing through the respective plural light-emitting elements, such that the light-receiving signals corresponding to the light-emitting levels of the respective light-emitting elements output from the light-receiving element in the semiconductor laser array and the light-emitting level designating signals respectively become equal to each other. The optical writing apparatus further includes a change-over switching device capable of changing over the plural semiconductor laser drive controlling devices to either one of them as a negative feedback loop, to which the light-receiving signal output from the light-receiving element in the semiconductor laser array is applied. The writing apparatus further includes an output timing determination device respectively determining an output timing of the change-over signal to the change-over switching device and another output timing of the control starting signal to the current controlling device in the respective semiconductor laser drive controlling devices. The output timing determination device further includes an output device for outputting the change-over signal to the change-over switching device and for outputting the control starting signal to the current controlling device in the semiconductor laser drive controlling device connected to the negative feedback loop formed by the changing-over operation of the change-over switching device, after at least the time corresponding to the operation delaying time of the change-over switching device elapses from that time point.
In a second aspect of the present invention there is provided a novel optical writing apparatus including a semiconductor laser array constructed with plural light-emitting elements and a single light-receiving element and plural semiconductor laser drive controlling devices. The plural semiconductor laser drive controlling devices respectively include a current controlling device, light-receiving signals corresponding to the light-emitting levels of the respective light-emitting elements output from the light-receiving element in the semiconductor laser array and the light-emitting level designating signals respectively become equal to each other. The optical writing apparatus further includes a change-over switching device and an output timing determination device. The output timing determination device further includes an output stopping device for stopping the outputting of the change-over signal to the change-over switching device, after finishing the control operation by use of the current controlling device in the semiconductor laser drive controlling device connected to the negative feedback loop formed by the changing-over operation of the change-over switching device among the current controlling devices in the respective semiconductor laser drive controlling devices.